The Passive Optical Network (PON) technology with the advantages of low-cost, multi-user access, ultra-long distance transmission, high transmission bandwidth etc., has gradually replaced the existing cable access network with the copper wire as a transmission medium, and becomes the mainstream broadband access network technology. At present, there are mainly two PON technologies, i.e., the Ethernet Passive Optical Network (EPON) and the Gigabit-Capable Passive Optical Network (GPON). Generally, the coverage of both PONs is 20 km, and the maximum branching ratio is 1:64 or 1:128. The maximum transmission distance and the splitting ratio are mainly limited by optical power budget of the PON network, while the optical power budget is mainly determined by two aspects: output power of an optical transmitter and sensitivity of the optical receiver used in an Optical Line Terminal (OLT) and an Optical Network Unit (ONU), which are determined by the existing technological level of the optical transceiver.
With the continuous increasing of the broadband access users, as well as the continuous expansion of the broadband service coverage rate, the next-generation PON has explicitly put forward requirements to extend the transmission distance and increase the branching ratio. This can significantly reduce the number of network nodes and reduce the cost of network deployment and maintenance. To achieve this purpose, the most simple and feasible method is to add a relay amplifier in a backbone fiber of an Optical Distribution Network (ODN), which has been explicitly pointed out in related standards of the next-generation PON.
The downlink signal of the PON is in a continuous mode, and the uplink signal uses a Time Division Multiplexing Address (TDMA) mode. Because the distances for uplink signals sent by different ONUs to reach the OLT are different, and the transmission optical powers per se between different ONUs are different, the power levels received by the OLT are different in various slots, which is referred to as “far-near problem”, and the existing standard stipulates that the maximum of the difference between uplink optical powers of different ONUs can be permitted to reach 15 dB. In order to receive bit streams correctly, the OLT receiver must adjust the decision level at the beginning of each burst slot, and such mechanism is referred to as Automatic Gain Control (AGC). The AGC time of the burst receiving module at the OLT side which is stipulated by the standard is very short, for example, the GPON stipulates that only 44 bits (less than 36 ns) are used for the AGC and clock synchronization. In most cases, the signal dynamic range of different ONUs needs longer AGC adjustment time than the stipulated time. And the next generation PON network with a higher rate will require shorter AGC time, and therefore, this problem will be more obvious. In order to reduce the gain adjustment range, the GPON suggests to use a power adjustment mechanism, i.e., the OLT indicates the ONU to adjust the power to make the powers of various ONUs reaching the OLT approximately equal. Although requirements on the AGC adjustment range of the OLT receiver are relaxed through the adjustment mechanism, this makes the ONU hardware more complex, and it is needed to add related control protocols between the OLT and the ONU.
In addition, future PONs will develop to a long-distance, multi-wavelength multiplexing direction. Related standards of the next generation PON have been proposed to add a reach extender unit on a trunk fiber link for converting the wavelength of an optical signal. Another use of the optical amplifier is as a pre-amplifier of a wavelength converter, while All Optical Wavelength Converter (AOWC) generally requires the input optical power to be within a certain range, which is referred to as a linear working area. If the input optical power exceeds the range, the waveform of the output optical signal will be distorted, or degrade the output extinction ratio, and therefore, when such type of optical device is applied in the uplink, the power difference between the uplink burst signals in different slots is also desired to be as small as possible.